Register SoapExtension in a code

SoapExtensionRegistration.cs

public static void RegisterSoapExtension(Type type, int priority, PriorityGroup group)
{
    if (!type.IsSubclassOf(typeof(SoapExtension)))
    {
        throw new ArgumentException("Type must be derived from SoapException.", nameof(type));
    }

    if (priority < 1)
    {
        throw new ArgumentOutOfRangeException(nameof(priority), priority, "Priority must be greater or equal to 1.");
    }

    // get the current web services settings...  
    WebServicesSection wss = WebServicesSection.Current;

    // set SoapExtensionTypes collection to read/write...  
    FieldInfo readOnlyField = typeof(System.Configuration.ConfigurationElementCollection).GetField("bReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
    readOnlyField?.SetValue(wss.SoapExtensionTypes, false);

    // inject SoapExtension...  
    wss.SoapExtensionTypes.Add(new SoapExtensionTypeElement(type, priority, group));

    // set SoapExtensionTypes collection back to readonly and clear modified flags...  
    MethodInfo resetModifiedMethod = typeof(System.Configuration.ConfigurationElement).GetMethod("ResetModified", BindingFlags.NonPublic | BindingFlags.Instance);
    resetModifiedMethod.Invoke(wss.SoapExtensionTypes, null);
    MethodInfo setReadOnlyMethod = typeof(System.Configuration.ConfigurationElement).GetMethod("SetReadOnly", BindingFlags.NonPublic | BindingFlags.Instance);
    setReadOnlyMethod.Invoke(wss.SoapExtensionTypes, null);
}